Overview
Routed overlay networks use BGP EVPN VXLAN technology to create scalable, flexible Layer 2 and Layer 3 campus fabrics. These networks enable efficient multihoming, streamlined route management, and support for robust redundancy and network segmentation.
The fundamentals of Layer 2 and Layer 3 networking with routed overlay in BGP EVPN VXLAN-based fabric networks remain unchanged from the traditional campus network deployment models. This section provides the key functional and operational components for building scalable BGP EVPN VXLAN fabric networks with EVPN multihoming.
The following figure shows the characteristics of an EVPN multihoming network with routed overlay fabric.
Hierarchical BGP control plane
In large-scale campus networks, the recommended deployment approach for controlled route management between EVPN multihoming peers and spines is a two-tier hierarchical BGP control plane. This design enables each BGP peering layer to manage domain-specific EVPN routes to support downstream non-blocking and all-active Layer 2 multipath network, and manage the advertisement of fabric network prefixes to enable secure global network access connectivity.
For more information, refer to the Hierarchical BGP Sessions chapter.
EVPN multihoming
The Layer 2 network with EVPN multihoming operation functions independently from the EVPN fabric core network. The pair of Cisco Catalyst 9000 series switches at each distribution layer form and manage EVPN multihoming within their respective redundancy group. These switches use a unified Layer 2 Ethernet Segment (ES) EtherChannel connection to downstream network devices. The EtherChannel connection can carry VLANs that are either mapped to routed overlay and advertised within the fabric core or continue to operate within the traditional underlay IP network.
Layer 2 broadcast network boundary
The EVPN fabric routed overlay networks provide a unified Layer 2 or Layer 3 network boundary similar to the traditional underlay campus networks. The Layer 2 bridge-domain and blast radius are contained between the Layer 2 access layer and directly attached EVPN multihoming-enabled distribution layer systems.
The pair of Cisco Catalyst 9000 series switches in the distribution layer network dynamically builds a Layer 2 VXLAN tunnel using ingress replication mode to extend Broadcast, Unknown Unicast and Multicast (BUM) traffic. With inbuilt loop detection and fast convergence techniques, the EVPN multihoming-enabled networks support secure and scalable Layer 2 networks over traditional STP-based networks.
Structured IP subnet and gateway redundancy
The Distributed Anycast Gateway (DAG) routed overlay networks follow the standard enterprise campus design principles with a VLAN or subnet per distribution block. Network administrators can define a structured IPv4 or IPv6 addressing plan to build scalable overlay networks.
A pair of Cisco Catalyst 9000 series switches operating in EVPN multihoming mode provides IP gateway redundancy by implementing a built-in gateway function for both IPv4 or IPv6 in each distribution block.
Spine-route policy
In large scale EVPN multihoming enabled fabric networks, controlled route management is enforced through route policies on leaf switches that are attached to spine switches. The iBGP or eBGP prefix advertisement to spine switches in the fabric core limits the advertising of IPv4 or IPv6 network prefixes, specifically EVPN route type 5 prefixes, while retaining unfiltered routing with iBGP peering sessions between leaf switches and remote ES switches. This design supports a non-blocking Layer 2 multipath EtherChannel that ensures scalability and efficient route distribution.
EVPN multihoming-enabled networks support additional overlay network types to stretch the IP subnets or Layer 2 flood-domains beyond a single distribution block by using the DAG technology. Routed overlay networks are recommended for better scale, performance, and resiliency.